Continuous web printing allows economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web of paper or other print media material is fed past one or more printing subsystems that form images by applying one or more colorants onto the print media surface. With this type of printing system, finely controlled dots of ink are rapidly and accurately propelled from the printhead onto the surface of a moving print media, with the web of print media often coursing past the printhead at speeds measured in hundreds of feet per minute. During printing, variable amounts of ink may be applied to different portions of the rapidly moving print media web, with drying mechanisms typically employed after each printhead or bank of printheads. Variability in ink or other liquid amounts and types or variability in drying times can cause print media stiffness and tension characteristics to vary dynamically for different types of print media, contributing to the overall complexity of print media handling and print media dot registration.
In some prior art web printing systems, such as the KODAK VERSAMARK VT3000 Printing System, the web media is slack when it enters the printing system and an “S-wrap” tensioning mechanism is used to add tension to the web media in preparation for feeding the web media into the rest of the system. S-wrap tensioning mechanisms provide an S-shaped media path where the web media is pulled across curved surfaces of tensioning shoes. Friction between the web media and the tensioning shoes introduce a tension into the web media.
The amount of tension introduced into the web media by an S-wrap tensioning mechanism will be a function of the coefficient of friction between the web media and the tensioning shoes. As a result, the amount of tension provided in a particular configuration can vary widely depending on the factors such as characteristics of the web media, operating speed and environmental conditions. Therefore, it is commonly necessary to manually adjust the geometry of the S-wrap tensioning mechanism (for example, by adjusting a wrap angle) to tune the system performance in accordance with the variation in these factors. Such manual adjustments can be time-consuming, and can be prone to operator error.
U.S. Patent Application Publication 2009/0101686 to Lane, entitled “Web processing apparatus,” discloses a web tensioning assembly configured to balance the tension across the width of a web. With this arrangement, the tension in the web media before and after the tensioning assembly will be the same. Therefore it is incompatible with applications where tension needs to be added to a slack web media.
U.S. Patent Application Publication 2011/0077115 to Dunn, entitled “System and method for belt tensioning,” discloses a method for adding tension to a belt which involves using a spring to apply a force to a tensioning roller. This configuration provides a controlled amount of tension throughout a closed belt, but cannot be used to add tension to a slack web media.
There remains a need for a tensioning mechanism for adding tension to a slack web that provides a consistent level of tension independent of varying media and environmental characteristics.